The invention relates to magnetic resonance (MR) imaging, and more particularly relates to MR imaging wherein the signal of a predetermined spectral component (e.g. fat, water) is saturated so as not to contribute to the image. In its most immediate sense, the invention relates to fat- or water-suppressed multislice MR studies, and to multislice MR studies in which the signal from at least one undesired spectral component is to be suppressed.
MR studies can be degraded by artifacts caused by blood flow or by reduced contrast caused by the presence of fat. To avoid such degradations, water- or fat-suppressed studies are carried out by saturating the water or fat signals. When this is done, the water or fat cannot contribute to the MR image. This saturation is in turn carried out by repeatedly using at least one saturation prepulse. The saturation prepulse(s) precede the imaging pulse sequence used to read out one or more lines of MR data.
When doing a fat-suppressed or blood-suppressed MR study of the multislice type, conventional water or fat suppression techniques can so greatly increase the time required to conduct the study that the study cannot properly be carried out. An example of such a study is a dynamic MR study of the liver and pancreas.
In a dynamic MR study of the liver and pancreas, Gd-DTPA is administered to the patient and the liver and pancreas are imaged in an MR scanner to gauge the rate at which the Gd-DTPA passes into the normal tissue and into abnormal regions within the organs. If the study is not conducted quickly, it cannot be conducted at all. There are two reasons for this. First, the Gd-DTPA becomes distributed quickly and discriminatory concentration differences may be lost. Second, a dynamic MR study of the liver and pancreas is best conducted while the patient holds his or her breath; if the patient breathes while the study is going on, respiratory motion will move the liver and the study will be degraded.
The pancreas adjoins the liver and becomes very conspicuous on fat-suppressed imaging. For this reason, an optimized upper abdominal study requires that the fat signal be suppressed. When fat-saturating saturation prepulses are used in a conventional manner, i.e. before acquisition of each line of MR data, a single breath hold lasts only long enough to acquire MR data for e.g. five slices.
The liver is a relatively large organ; perhaps twenty slices of the patient's body are required to completely image it. It would therefore take at least four sequential breath hold periods to conduct a complete MR liver study, and this is too long for a dynamic uptake study. Therefore, the need for a fat-saturating MR pulse sequence makes it impossible to conduct a comprehensive dynamic MR study of a patient's liver and pancreas.
The invention proceeds from a realization that conventional use of saturation prepulses for water and fat takes up valuable additional time over and above the time required to achieve the desired objective of saturation. Water and fat spins have finite T1's, i.e. take a certain time to recover their original longitudinal magnetization after being nutated by an RF pulse. When saturation prepulses succeed each other too rapidly, the nutated spins have recovered little if any of their original longitudinal magnetization before being subjected to the next saturation. As a result, too-frequently repeated saturation prepulses have little physical effect on the water and fat spins.